Rapid progress in the functional characterization of proteins involved in signal transduction and cell cycle regulation has highlighted their importance for appropriate signal responses and regulation of cell differentiation, cell growth and cell division. We are working on the crystal structure of a complex of cyclin dependent kinase 2 with cyclin A. CDKs are central to cell cycle regulation and can only be activated after complex formation with a cyclin. Hence a complex structure will provide important structural information about the activation process of CDK2.